Optical computing and optical digital signal processing systems typically incorporate some form of optical switching alements. While hybrid electronic-Optical devices, such as Self Electro-optic Effect Devices (SEED) have been applied to optical digital processing, all-optical digital systems offer the possibility of very high integrity systems, wide bandwidth, high speed, bulk processing, and elimination of optical-to-electronic interfaces. One of the extant mechanisms for all-optical switching devices is the use of optical bistability, in which an illuminated device made with a nonlinear optical material can exhibit either of two constant light intensities as the output of the device for a given light intensity as the device input. Optical bistability is a steady-state effect which requires the input optical fields to be constant for a time period on the order of several induced dipole-dephasing times. The nonlinearity may be absorptive or dispersive and feedback is generally provided by locating the material in an optical cavity.
Another mechanism for accomplishing all-optical switching is the optical pumping of a population of effectively two-level systems from one level to the other level. In thermodynamic equilibrium, population in excited quantum-mechanical energy levels tends to drop to the lowest energy level that is available. Population can be pumped from lower energy levels to higher energy levels. The population then decays through the ladder of levels, by radiative or non-radiative transitions, to the lowest available energy level. While most levels quickly decay to lower-energy levels, there may be long-lived excited levels with lifetimes that are long compared to a time of interest, e.g., an optical pulse width. A population inversion exists when the population in an excited level is larger than the population in the lower level, or levels, to which it is coupled.
When other energy levels can be neglected, a long-lived level with a radiative transition to a long-lived, lower energy level can be modeled as a dipole-coupled, two-level, quantum mechanical system. In this model, the lower level iB typically called the ground state and the upper level is typically called the excited state, even though higher and lower energy levels may exist in the full ladder of levels from which the two-level system is extracted. Examples of such effectively two-level systems are electronic energy levels in atoms, vibrational energy levels in molecules and editions near the band edge in semiconductors.
In incoherent pumping, population is typically excited indirectly to a long-lived excited level, either by collisions or by excitation to short-lived levels which decay to a long-lived excited level. Lasing can occur when this metastable level has a radiative transition to a lower energy level and the population becomes inverted. The principal methods for incoherent pumping are electric discharge excitation and optical excitation with a flashlamp.
Coherent pumping directly excites a radiative transition between long-lived levels. An ultra-short coherent pulse, resonant with the transition frequency, fully inverts a collection of two-level systems when the pulse area is an odd multiple of u, where the pulse area is the time integral of the field strength, expressed in angular frequency units. In adiabatic inversion, the field strength is kept constant and the detuning from resonance is swept slowly through zero, inverting the population. However, these methods, based on noninteracting two-level systems, are not valid in dense media because they neglect the effect of dipole-dipole interactions which occur in dense collections of two-level systems.